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Thread: Lav trap on larger trap arm: actual problem?

  1. #1

    Default Lav trap on larger trap arm: actual problem?

    I have a 2" trap arm for a lav, and just realized a 1-1/4" lav drain can't
    use a 2" trap. The UPC shows the trap arm size must match the trap
    size, but does anyone know what problem a 1-1/4" trap on a 2" trap
    arm would cause? Self siphoning should actually be less than a smaller
    sized arm, so I'm not sure what the problem would be if I just go ahead
    and put the 1-1/4" trap on.

    Dave

  2. #2
    Master Plumber Redwood's Avatar
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    Default

    Is it the I.D. of the pipe that measures 2"?

  3. #3
    Moderator & Master Plumber hj's Avatar
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    Default trap

    Forget what you think the UPC says about arm and trap sizes. Install the trap. If you have the proper venting it will not siphon regardless of the arm size, and if you screwed up the venting, then it WILL siphon regardless of the size.

  4. #4

    Default Re: Trap

    So it sounds like all I have to worry about is inspector confusion
    on the final.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Default trap

    You are confused. The inspector will not be.

  6. #6

    Default Re: Trap

    Oh. So how am I confused?
    And what convinces you the inspector will not be?

  7. #7
    Moderator & Master Plumber hj's Avatar
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    Default confusion

    Because you are misinterpreting the code, and the inspector knows what the code says.

  8. #8

    Default Re: Trap

    Perhaps you could be more specific how I am misinterpreting the code.

    The rough inspection for this construction was done under the '97 UPC.

    Paragraph 1003.3 states, The size (nominal diameter) of at trap for a
    given fixture shall be sufficient to drain the fixture rapidly, but in no case
    less than nor more than one (1) pipe size larger than given in Table 7-3.
    The trap shall be the same size as the trap arm to which it is connected"

    Table 7-3 shows 1-1/4" for a single lav, so it looks like the trap can be
    no bigger than 1-1/2". But since the trap arm was roughed at 2", the
    end of paragraph 1003.3 would seem to insist the trap needs to be 2".

    Of course, the lav drain is only 1-1/4".

    The trap arm was roughed to 2" per instructions of the inspector.

    Dave

  9. #9
    Moderator & Master Plumber hj's Avatar
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    Default arm

    The trap arm was roughed to 2" per instructions of the inspector

    That is the ONLY way I have installed trap arms for 55+ years and most of the time the trap IS 1 1/4". I know it is OK, the inspector knows it is OK, only you seem to have a problem with it.

  10. #10
    Retired Defense Industry Engineer jadnashua's Avatar
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    Default

    In plumbing, you can always go from smaller to larger drain pipes...just not the other way.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  11. #11
    Plumber Winslow's Avatar
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    Default

    If the distance from the trap to vent is too far for 11/2 (31/2 feet) then it is common to run the trap arm 2" then reduce it at the fixture to 11/2. You can then use either a 11/2 or 11/4 trap.

  12. #12

    Default Re: Trap

    >The trap arm was roughed to 2" per instructions of the inspector

    >That is the ONLY way I have installed trap arms for 55+ years and most of the time >the trap IS 1 1/4". I know it is OK, the inspector knows it is OK, only you seem to have >a problem with it.

    Not exactly. This particular inspector also told me I needed a 2" trap. After the
    rough inspection, I noticed in the code that I couldn't go over a 1-1/2" trap
    for the 1-1/4" lav drain, so he basically misdirected me. The reason I had to
    change any DWV in this area on my rough was because I had put two lavs
    on the same trap arm. As far as I could tell, this was allowed by the code,
    but he had never seen it before.

    Another one of the inspectors in my area told me to put in a horizontal wet vent
    in a particular location. Since that was clearly against the code I was working
    under at the time, I had to get around the problem by getting an approval by
    the plumbing plan checker (he knows the code well) on my layout before
    the next inspection. Inspectors are human, and some know more than others.

    While I could think of no physical reason there would be a problem with a
    1-1/4" trap on a 2" trap arm, it is against the letter of the code, so I wanted
    to run it by some of you to make sure I wasn't missing something.

    Thanks,

    Dave

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